The sinus, or paranasal sinus, as it is commonly known, is a parasitic rat that inhabits your face. It is believed that they are implanted at birth and live with you your entire life, rather like your conscience, or your parents. There are several species of sinuses, the maxillary sinus, the frontal sinus, the ethmoid sinus and the sphenoid sinus.
The maxillary sinus is the most sneaky sinus ever to exist. The first, and possibly most sneaky thing it has done is taken a fake id, and maskerades as an antra. It inhabits your cheekbone, just under your eyes. It is believed that they sit there, control where your eyes go. So, if caught lookin' at something you are not supposed to, just blame it on your antra. The maxillary sinus (Scientific name: Wittius scientific-nameicus) inhabits all people, and was responsible for your fathers death.
The frontal sinus, conversely, is the most blunt sinus, inhabiting the area of your forehead that, in an intelligent life-form, would house your brain. It is responsible for cheese, which is the offspring of the frontal sinus and a brain. Frontal sinuses are so blunt that it is possibly to hammer in posts with them. Very small, easily broken posts. The frontal sinus is responsible for all the boring involutary stuff you do, like breathe, beat your heart, killing people.
The Ethmoid sinus lives between the nose and the eyes, backwards into the skull. It is like the retarded cousin of Frontal sinus, but is rather more materialistic, concerned with 'bling'. Never look into their eyes. Ever!.
The sphenoid sinus is the retarded cousin of the Ethmoid sinus, and does not therefore warrant a capital letter, or multiple sentences, or even a picture.
The sinuses probably fulfil a number of functions. Noone knows for sure. In general they provide excuses for inappropriate behaviour and make it hurt less when people punch you in the face for innappropriate behaviour.
Sinuses always suffer from sinusitis, which is the belief that one is not a sinus. Anyone who has sinusitis is a sinus. Morons.